The present invention relates to a method and device in the regulation of the headbox of a paper/board machine, by means of which method and device it is possible to reliably act upon the grammage profile of the paper reliably across the width of the paper/board web and advantageously, it is also possible to act upon the fiber-orientation profile of the paper/board web across the width of the paper/board web.
As is known from the prior art, the discharge flow of the pulp suspension out of the headbox should have a uniform velocity in the transverse direction of the paper/board machine and. A transverse flow produces distortion of the fiber orientation and adversely affects the quality factors of the paper produced, such as anisotropy of strength and stretch. The level and variation of anisotropy in the transverse direction also affect the printing properties of the paper. In particular, it is an important requirement that the main axes of the directional distribution, i.e. orientation, of the fiber mesh in the paper coincide with the directions of the main axes of the paper and that the orientation is symmetric in relation to these axes.
At the edges of the pulp-flow duct in the headbox, a smaller amount of pulp flows. This edge effect produces a very strong linear distortion in the fiber-orientation profile. Profile faults in the turbulence generator of the headbox usually produce a non-linear distortion in the fiber-orientation profile inside the lateral areas of the flow ducts.
Attempts have been made to compensate for an unevenness of the grammage profile arising from the drying-shrinkage of paper by means of a crown formation of the slice, so that the slice is thicker in the middle of the pulp jet. However, it is a phenomenon in the manufacture of paper that when the paper web is dried, it shrinks in the middle area of the web to a lower extent than in the lateral areas. The shrinkage is typically in the middle of the web about 4% and in the lateral areas of the web from about 5% to about 6%. This shrinkage profile produces a corresponding change in the transverse grammage profile of the web. As a result of the shrinkage, the dry grammage profile of a web whose transverse grammage profile was uniform after the press is changed during the drying so that, in both of the lateral areas of the web, the grammage is slightly higher than in the middle area. As known from the prior art, the grammage profile has been regulated by means of the profile bar so that the profile bar of the headbox is kept more open in the middle area of the headbox than in the lateral areas of the headbox.
By means of this type of arrangement, the pulp suspension is forced to move towards the middle area of the web. However, there circumstances further affect the alignment of the fiber orientation. The main axes of the directional distribution, i.e. orientation, of the fiber mesh should coincide with the directions of the main axes of the paper, and the orientation should be symmetric in relation to these axes. In the regulation of the profile bar, a change in the orientation is produced as the pulp suspension flow receives components in the transverse direction.
Regulation of the lip of the headbox also produces a change in the transverse flows of the pulp jet even though the objective of the regulation is exclusively and only to affect the grammage profile, i.e. the thickness profile of the pulp suspension layer that is fed. Thus, the transverse flows have a direct relationship with the distribution of the fiber orientation.
From the prior art, specific devices are known by whose means attempts are made to regulate the fiber orientation, and other devices are known by whose means attempts are made to regulate the grammage profile of the web. However, when the grammage profile is regulated in a prior art devices by means of the profile bar, the fiber orientation in the web is unavoidably also affected at the same time.
From the prior art, a method is known in the headbox of the paper machine to control the distortion of the fiber orientation in the paper web. In such a method, medium flows are passed into lateral passages placed at the level of the turbulence generator of the headbox, and, by regulating the magnitudes and the mutual proportions of these flows, the transverse flows of the pulp suspension are affected, and thereby the distortion of the fiber orientation is regulated. By means of the flows introduced into the lateral passages, a transverse flow velocity is produced which compensates for the distortion of the fiber orientation.
From the assignee's Finnish Patent Application No. 884408 of earlier date (corresponding to the assignee's U.S. Pat. No. 5,022,965, the specification of which is hereby incorporated by reference herein), a method is known in the headbox of a paper machine to control the distribution of the fiber orientation of the paper web in the transverse direction of the machine. In the method described in FI '408, the transverse velocity component of the discharge jet is regulated by appropriately aligning the turbulence tube of the turbulence generator.
By means of the above mentioned prior art methods for controlling the fiber orientation in the paper web, it is usually possible to control only the linear distortion profiles. The prior art methods are thus suitable for the control of the fiber orientation, but, when they are used, commonly even a large non-linear residual fault remains in comparison with an even distribution of the orientation. The prior art methods are also well suitable for basic regulation of the distortion of the orientation. However, by means of the prior art methods, it is not possible to regulate individual faults which may occur in the orientation in the middle area of the web and which arise, e.g., from defects in the pipe system of the turbulence generator.
A number of methods are also known for the regulation of the profile bar, in which, while the grammage profile is measured, the position of the profile bar in the headbox of the paper machine is changed. In addition, by means of the profile bar, the thickness of the pulp suspension discharged onto the wire, and thereby, the grammage of the paper web, are affected. In the manner described above, this regulation produces faults in the orientation because, by means of the regulation, the flow is throttled elsewhere, whereby components of transverse velocity are produced in the flow.
In the invention described in the assignee's Finnish Patent No. 50,260 (corresponding to U.S. Pat. No. 3,791,918), the headbox has been divided across its width, in a direction transverse to the main flow direction, into compartments by means of partition walls. In this headbox, in an individual compartment, there is at least one inlet duct for the passage of a component flow to feed diluting water into the pulp flow.